Wafer laser crystal

ABSTRACT

The present invention concerns a laser with a laser crystal in wafer form. In order to provide a laser apparatus with laser materials in wafer form which are improved over the state of the art, and a process for the production of improved laser materials in wafer form for such laser apparatuses, it is proposed in accordance with the invention that the laser crystal is of the chemical composition M I R III (WO 4 ) 2 , wherein M 1  stands for an alkali metal, R III  stands for a lanthanide and X stands for a laser-active doping substance.

The present invention concerns a laser having a laser crystal in waferform and a process for the production of a laser crystal in wafer form.

Laser light, that is to say light which is spatially and temporallycoherent has found uses in the meantime in many fields. Thus lasertechnology is used for example in the areas of medicine, productiontechnology, measurement and testing procedures and environmentalprotection. The demands on laser technology in those areas areconstantly rising and there is a great need for more powerful and moreefficient lasers which operate reliably, afford a high level of beamquality and which are to be operated with the greatest possible freedomfrom trouble and maintenance.

Inter alia solid state, gas and liquid lasers as well as lasers usingsemiconductor materials can be used for the production of laser light.

In regard to solid state lasers, besides the traditional rod lasers,wafer lasers have now been known for some time. Wafer lasers involveusing a laser crystal in wafer form. The layer thickness of the lasercrystal is generally in a range of some tenths of a millimetre to somemillimetres and is thus markedly reduced in comparison with the layerthickness of laser crystals in rod form of conventional rod lasers(d=about 10 cm). The diameter of the laser crystals in wafer form isgenerally about 10 mm.

The concept of the wafer laser is based on a laser medium in wafer form,which is mounted on and connected to a cooling element—which isgenerally liquid-cooled. The rear side of the laser wafer is cooled atone side by the cooling element. The area cooling effect at the rearside of the very thin laser crystal gives rise to temperature gradientspredominantly in the direction of the laser beam and therefore havescarcely any influence on the quality of the laser beam. That is incontrast to the conventional rod laser in which the thermally inducedchanges have a considerably more severe adverse influence on theproperties of the laser medium, with the laser beam beingcorrespondingly more severely optically distorted. Thermal lens effectsand thermally induced birefringence are also comparatively reduced inthe wafer laser.

On the side connected to the cooling element the laser medium in waferform is frequently provided with a reflective coating. For the purposesof connecting the laser wafer to the cooling element, the arrangementoften has a soft, thermally conductive intermediate layer which cancushion thermal deformations of the laser wafer which occur in thepumping operation or in the production of laser light and can absorbheat from the laser wafer and transmit it to the cooling element.

Various chemical compositions have already been tested as materials forwafer lasers. The most widespread is ytterbium-dopedyttrium-aluminium-gamet (Yb:YAG) of the chemical formula Yb:Y₃Al₅O₁₂. Inthat material the yttrium-aluminium-garnet represents the neutral basiclattice of the laser material which is not involved in the actual laserprocess. The constituents (atoms, ions and molecules) which are crucialfor laser light emission, the so-called laser-active substances, areincorporated into the basic lattice of a laser material. In the case ofthe Yb:YAG the laser-active substance is the ytterbium.

Yb:YAG has good mechanical properties which allow the commercialproduction of wafers of diameters in the range of 5 to 25 mm and ofwafer thicknesses of about 300 μm. It will be noted however that thelaser-specific properties of the Yb:YAG are markedly surpassed by othermaterials. For example ytterbium-doped potassium-yttrium-tungstate(Yb:KYW) of the chemical formula Yb:KY(WO₄)₂ is known for its highabsorption and emission cross-sections. However production of thepreferably very thin laser wafers from the laser material Yb:KYW is inpractice extremely difficult as that material is of relatively lowhardness and has only little mechanical strength.

The following problems frequently occur in operation of laserapparatuses with conventional laser materials with good laser-specificproperties in wafer form. Thus the thermally induced deformationphenomena referred to in the opening part of this specification, even inthe case of laser wafers mounted on soft intermediate layers, notinfrequently result in flaws or fractures in the crystals. The mountingof crystal wafers on a cooling liquid film is also critical and oftenresults in destruction of the crystal, particularly with very thinwafers.

In laser technology therefore there is a need for laser apparatuses withlaser materials in wafer form, which enjoy very good mechanicalproperties like the widespread Yb:YAG and which at the same time havemarkedly better laser-specific properties in comparison with Yb:YAG. Inparticular it is desirable to provide laser materials having highabsorption and emission cross-sections, from which wafers which are asthin as possible can be produced, which can be used in laserapparatuses, which can be permanently employed therein and which arepossibly also interchangeable, without fracturing or breaking.

Consequently the object of the present invention is to provide a laserapparatus with laser materials in wafer form, which are improved overthe state of the art, and a process for the production of improved lasermaterials in wafer form for such laser apparatuses.

In accordance with the invention that object is attained by the use of alaser crystal of the chemical composition M^(I)R^(III)X(WO₄)₂, whereinM^(I) stands for an alkali metal, R^(III) stands for a lanthanide, and Xstands for laser-active ions with which the material is doped, andwherein the material is provided in the form of a wafer.

The basic lattice structure of that material is M^(I)R^(III)X(WO₄)₂,wherein R^(III) stands for at least one element from the group oflanthanides which includes the elements lanthanum (La), cerium (Ce),praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm),europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium(Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu). Thatbasic lattice is doped with active laser ions, the active laser ionspreferably being selected from Yb³⁺, Nd³⁺,Er³⁺, Ho³⁺, Tm³⁺and Pr³⁺.

In preferred embodiments the alkali metal (M^(I)) is selected fromlithium, sodium, rubidium and caesium.

In a particularly preferred embodiment M^(I) is sodium (Na).

Preferably the laser material used in apparatuses according to theinvention is congruent-melting. The term congruent-melting material isused here to denote a material comprising a compound which does notalready dissociate into its components below its melting point butbreaks down into its components only at the moment of melting, solid andliquid phases involving the same equilibrium composition.

By means of a number of tests it was possible to demonstrate that thetungstate material provided in the apparatus according to the inventionnot only has very good laser- specific properties but also excellentmechanical properties which make it possible to also use very thin lasermedia which have sufficient mechanical strength to be suitable for usefor the usual applications as a wafer laser. This means that processingof the material of the aforementioned composition to provide wafers ofvery small thickness is possible, in which respect wafers of very smallthickness can be easily cut out from a crystal body comprising one ofthe claimed materials and then polished without the material beingdamaged in that procedure. Thus, when polishing such wafers, markedlyfewer edge breakages occur than when polishing conventional materialsand the anisotropic properties of the polished surfaces are very muchless pronounced than for example with Yb:KYW.

In preferred embodiments of the invention the laser material wafer ispreferably of a thickness of <3 mm. In further preferred embodiments thethickness of the laser wafer is between 0.5 μm and 1 mm and in the caseof special embodiments of this invention it is in a range of between 5and 250 μm.

The term wafer is used here to denote a body whose mean thickness is amultiple smaller than its length and width. In that respect the externalshape of the body is basically irrelevant. Thus that definition embracesbodies having a triangular, rectangular, polygonal or round base surfaceand also such bodies whose thickness is not constant over the entirebody. In a narrower sense the term wafer is used herein to denote a bodycorresponding to the above-mentioned definition with a symmetrical basesurface, wherein the surfaces of the top side and the undersiderespectively of the wafer are planar to slightly curved.

Preferably the crystal material according to the invention is cut intocircular to oval wafers. In the case of circular wafers, it isparticularly preferred if the ratio of the diameter of the wafer D tothe thickness of the wafer L is greater than 4.9. In that respect it isparticularly advantageous if the diameter of the wafer D is in the rangeof between 1 and 51 mm, preferably in the range of between 2 and 30 mmand particularly preferably in the range of between 3 and 20 mm.

In order to obtain surfaces which are as flat as possible on the laserwafer, it is particularly preferred if at least a part of the surface ofthe wafer is polished. Further preferred embodiments are characterisedin that the surface of the wafer is at least partially de-reflected orbloomed or provided with a reflecting coating. The surface of the waferis preferably de-reflected on the side of the wafer, which is inopposite relationship to the cooling element. On the side towards thecooling element the laser wafer is preferably provided with a coatingwhich is highly reflective both for the pump wavelength and also for theemitted laser wavelength.

In a special embodiment of the present invention the lanthanide in theabove- mentioned chemical composition is gadolinium (Gd). Preferablythose materials are doped with the active laser ions Yb³⁺or Nd³⁺.

A particularly preferred embodiment of the laser according to theinvention uses a material of the general formula NaGd_(1−x)Yb_(x)(WO₄)₂,wherein x is preferably of a value of between 0 and 1. A value for xbetween 0.01 and 0.4 is particularly preferred and a value for x ofbetween 0.05 and 0.25 is especially preferred.

Another preferred embodiment of the laser according to the inventionuses a material with a Nd³⁺doping and is described by the generalformula NaGd¹⁻Nd_(x)(WO₄)₂, wherein x is preferably of a value ofbetween 0 and 0.2. With that material, a value for x of between 0.001and 0.1 is particularly preferred and a value for x of between 0.005 and0.05 is especially preferred.

In a further specific embodiment of the invention the laser materialcontains lanthanum (La) as the lanthanide. Preferably those materialsinvolve a doping with ytterbium (Yb³⁺) or neodymium (Nd³⁺). Aparticularly preferred embodiment of that material is doped withytterbium and is described by the general formulaNaLa_(1−x)Yb_(x)(WO₄)₂, wherein x is preferably of a value of between 0and 1. It is particularly preferred for the value of x to be between0.01 and 0.4 and a value for x which is especially preferred is between0.05 and 0.25.

A further preferred material of the aforementioned kind involves adoping with neodymium and its general formula is NaLa_(1−x)Nd_(x)(WO₄)₂.In this embodiment the value for x is preferably between 0 and 0.2 andis particularly preferably between 0.001 and 0.1. A value for x ofbetween 0.005 and 0.05 is especially preferred.

It will be appreciated that it is also possible to use liquid crystalswhich are segmented or assembled by means of bonding. In that way it ispossible to use liquid crystals with undoped ends or end layers. Whenusing such composite laser materials which comprise a doped segment ofthe above-mentioned chemical composition (e.g.: NaGd_(1−x)Nd_(x)(WO₄)₂)and an undoped segment on the basis of the corresponding chemicalcomposition (for example: NaGd(WO₄)₂) ground state absorption andthermal lens effects can be produced. In addition the coatings remain atthe undoped ends at low temperature and are therefore not exposed to anytroublesome thermally induced stresses. The scatter loss induced by theinterface between doped and undoped segments is generally negligible.

Such composite crystals can further increase the efficiency of thinwafer lasers in many cases. The enhanced mechanical stability ofmaterials of the chemical compositions described herein considerablyexpands the possible options in terms of production of theabove-mentioned bonded or segmented laser materials.

In order to obtain the tungstate material provided in the laserapparatus according to the invention in crystal form, preferably in theform of a single crystal, there is also provided a process for theproduction of such a crystalline material.

To produce that material, in the process according to the invention, acrystal is grown from a melt of the chemical compositionM^(I)R^(III)X(WO₄)₂ in accordance with the Czochralski process, whereinM^(I) is an alkali metal, preferably lithium, sodium, rubidium orcaesium, particularly preferably sodium, R^(III) is at least onelanthanide and X are laser-active ions. Preferably the lanthanide isgadolinium (Gd) or lanthanum (La). Preferably ytterbium (Yb) orneodymium (Nd) are used for the doping operation as laser-active ions.In these preferred embodiments of the process according to the inventionthe growth temperature is about 1,200 to 1,300° C.

Following growth of the crystal, which is generally concluded afterabout 14 days, the crystal axes (in the case of Yb-doped NaGd(WO)₄) ofthe grown crystal are determined, and then a rod of the desired diameteris bored out of the grown crystal, corresponding to the crystal axes.Wafers of the desired layer thickness are then cut from that crystal rodand the resulting wafers are polished.

In a particularly preferred embodiment of the laser according to theinvention the laser material is optically pumped with light in awavelength range of 390 to 2,100 nm. In that case preferably laseremissions in the wavelength range of between 400 and 3,000 nm areproduced.

For the purposes of the original disclosure it is pointed out that allfeatures as can be deduced by a man skilled in the art from the presentdescription and the claims, even if they are described specifically onlyin conjunction with certain further features, can be combined bothindividually and also in any combinations with others of the features orgroups of features disclosed herein, insofar as that has not beenexpressly excluded or technical factors make such combinationsimpossible or meaningless. A comprehensive and explicit representationof all conceivable combinations of features is not set forth here onlyfor the sake of brevity and readability of the description.

The following examples may be considered by way of example of thepossible combinations arising herefrom, such examples also describingadditional features and further embodiments of the present invention.

EXAMPLE 1

Growth of NaGd_(1−X)Yb_(X)(WO₄)₂ crystals with x=0.01−1 from a melt ofthe same composition in accordance with the Czochralski process.

The growth temperature is approximately 1250° C. The melt is prepared ina crucible of iridium or platinum, in which respect platinum crucibleshave the advantage that operation can be conducted in the presence ofambient air. A seed crystal is introduced into the liquid melt and thetemperature is so adjusted that the seed and the melt are inequilibrium. The crystal is then drawn slowly out of the melt. Thediameter of the crystal can be controlled by way of a weighing device.Crystals of a diameter of up to 40 mm and a length of up to 80 mm can beproduced in 7 days. After the growth procedure those crystals are cooledto 20° C. in about 2 days. They are colourless and can be subjected tofurther processing directly.

The crystal axes are determined prior to the first step of furtherprocessing. That can be effected by means of optical andX-ray-photographic processes. The desired orientations for wafers areperpendicular to the c- or a-axes. After the operation of boring out arod with the desired axis and of the desired diameter the wafers aresawn off to give a desired thickness (for example 0.35 mm). A set ofthose wafers is ground and polished with a double-sided polishingprocess. In that case the final dimension is achieved with a desiredthickness (for example 130 μm) and the desired planarity (for exampleλ/8 with an emission wavelength of λ=663 nm). If desired specialpolishes can be implemented on individual wafers in order to producesurfaces with an extremely low defect density.

Particularly good mechanical properties could be observed for xapproximately equal to 0.05.

EXAMPLE 2

Growth of NaGd_(1−X)Nd_(x)(WO₄)₂ crystals with x=0.001−0.2 from a meltof the same composition in accordance with the Czochralski process, asdescribed in detail for example 1).

EXAMPLE 3

Growth of NaLa_(1−x)Yb_(x)(WO₄)₂ crystals with x=0.01−1 from a melt ofthe same composition in accordance with the Czochralski process, asdescribed in detail for example 1).

EXAMPLE 4

Growth of NaLa_(1−x)Nd_(x)(WO₄)₂ crystals with x=0.001−0.2 from a meltof the same composition in accordance with the Czochralski process, asdescribed in detail for example 1).

1. A laser with a laser crystal in wafer form, characterised in that thelaser crystal is of the chemical composition M^(I)R^(III)X(W0 ₄)₂,wherein M¹ stands for an alkali metal, R^(III) stands for a lanthanideand X stands for a laser-active doping substance.
 2. A laser accordingto claim 1 characterised in that M^(I) is either lithium, sodium,rubidium or caesium.
 3. A laser according to claim 1 or claim 2characterised in that X is either Yb, Nd, Er, Ho, Tm or Pr.
 4. A laseraccording to one of claims 1 to 2 characterised in that the wafer is ofa thickness L of less than 3 mm.
 5. A laser according to one of claims 1to 2 characterised in that the ratio of the diameter D of the wafer tothe thickness L of the wafer is greater than 4.9.
 6. A laser accordingto one of claims 1 to 2 characterised in that the diameter D of thelaser crystal wafer is in the range of between 1.0 and 51.0 mm.
 7. Alaser according to one of claims 1 to 2 characterised in that one sideof the wafer is at least partially provided with a reflective coating.8. A laser according to one of claims 1 to 2 characterised in thatR^(III) stands for gadolinium (Gd), wherein X is Yb or Nd.
 9. A laseraccording to claim 8 characterised in that the laser crystal is of thegeneral formula NaGd_(1−X)Yb_(X)(W0 ₄)₂, wherein x is of a value ofbetween 0 and
 1. 10. A laser according to claim 8 characterised in thatthe laser crystal is of the general formula NaGd_(1−X)Nd_(x)(W0 ₄)₂,wherein x is of a value of between 0 and 0.2.
 11. A laser according toone of claims 1 to 2 characterised in that R^(III) stands for La;wherein X is Yb or Nd.
 12. A laser according to claim 11 characterisedin that the laser crystal is of the general formulaNaLa_(1−x)Yb_(x)(WO₄)₂, wherein x is of a value of between 0 and
 1. 13.A laser according to claim 10 characterised in that the laser crystal isof the general formula NaLa_(1−x)Nd_(x)(W0 ₄)₂, wherein x is of a valueof between 0 and 0.2.
 14. A laser according to one of the claims 1 to 2characterised in that the laser crystal comprises at least two portionsof different chemical compositions; wherein one portion is not dopedwith laser-active ions.
 15. A laser according to one of claims 1 to 2characterised in that there is provided a means for cooling one side ofthe laser crystal.
 16. A laser according to one of claims 1 to 2characterised in that there is provided a means for optically pumpingthe laser crystal with light of a wavelength in a wavelength range offrom 390 to 2,100 nm.
 17. A process for the production of a lasercrystal in wafer form comprising the following steps: i) growing acrystal out of a melt of the chemical composition M^(I)R^(III)X(WO₄)₂,wherein M^(I) stands for an alkali metal. R^(III) stands for alanthanide, and X stands for a laser-active doping substance, ii)determining the crystal axes of the grown crystal, iii) boring out a rodfrom the grown crystal in the direction of a crystal axis, and iv)cutting off wafers of desired thickness from the crystal rod.
 18. Aprocess according to claim 17 wherein at least parts of the surface ofthe wafers are polished.
 19. A laser according to claim 4, characterizedin that the wafer is of a thickness L of between 0.5 μm and 1 mm.
 20. Alaser according to claim 19, wherein L is of between 5 and 250 μm. 21.The laser according to claim 5, characterized in that the ratio of thediameter D of the wafer to the thickness L of the wafer is greater than7.5.
 22. A laser according to claim 6, characterized in that thediameter D of the laser crystal wafer is in the range of between 2 and30 mm.
 23. A laser according to claim 22, wherein D is between 3 and 20mm.
 24. A laser according to claim 9, wherein X is of a value of between0.01 and 0.4.
 25. A laser according to claim 24, wherein X is of a valueof between 0.05 and 0.25.
 26. A laser according to claim 10, wherein Xis of a value of between 0.001 and 0.1.
 27. A laser according to claim26, wherein X is of a value of between 0.005 and 0.05.
 28. A laseraccording to claim 12, wherein X is of a value of between 0.01 and 0.4.29. A laser according to claim 28, wherein X is of a value of between0.05 and 0.25.
 30. A laser according to claim 13, wherein X is of avalue of between 0.001 and 0.1.
 31. A laser according to claim 30,wherein X is of a value of between 0.005 and 0.05.
 32. A laser accordingto claim 2, wherein M′ is sodium.
 33. The process of claim 17, whereinM′ stands for sodium; wherein R″′ stands for gadolinium; and wherein Xstands for Yb.